One means of minimizing the impact of corrosion on metal surfaces has been to coat the surface with paint. The paint acts as a barrier between the metal surface and the environment and thus helps to prevent or at least minimize corrosion of the metal surface. However, one problem associated with this solution is that paint does not always adhere properly to the metal surface. The result may be peeling, cracking, blistering, or flaking of the paint, thus rendering the substrate metal surface again subject to corrosion.
The need for applying protective coatings to metal surfaces for improved corrosion resistance and paint adhesion is well known in the metal finishing and other metal arts. One attempt to alleviate the problem of poor adhesion of paint to metal surfaces has been to subject the metal substrate to a treatment which is known as phosphating, i.e. a process by which the metal surfaces are treated with chemicals which form a metal phosphate conversion coating on the metal surface. Such treatment typically assists in rendering the metal surface less subject to corrosive attack and, at the same time, in rendering the surface more suitable for application of paint. The resulting bond between the metal surface and the paint is thus greatly improved. However, phosphate baths require that precise formulations be maintained and that the processing procedures and conditions of operation be controlled within narrow limits. The phosphating process also requires that the metal surface be given two rinses subsequent to the phosphating bath, the first being a water rinse and the second being a passivating solution rinse which further enhances the corrosion resistance and adhesion characteristics of the coating. Traditionally, conversion coated metal surfaces have been given a second rinse with a solution containing a hexavalent chromium compound.
Lindert, in U.S. Pat. No. 4,433,015, teaches that, because of the toxic nature of hexavalent chromium compounds, expensive treatment equipment must be used to remove chromates from water effluent to prevent the pollution of rivers, streams and drinking water sources. Hence, in recent years there have been research and development efforts directed to discovering effective alternatives to the use of such post-treatment solutions. Lindert teaches that an alternative to the hexavalent chromium compound is a polymer having phenol groups attached along an ethylenic polymer backbone. The phenol groups may have a amine substituent which may further comprise hydroxy-alkyl groups. The polymer, made water soluble through neutralization of the amine moiety with organic acid, may be employed in an acidic or basic solution. It is also taught by Lindert that this solution, in addition to being used as a post-phosphate rinse, may be used to treat previously untreated metal surfaces including aluminum and zinc.
Frank et al, in U.S. Pat. No. 4,466,840, teach that there exists a need for a simple means to achieve results similar to that obtained with the phosphating process without the complexity of such a treatment. As an alternative to such phosphating treatment, Frank et al propose employing hydroxybenzylamines, preferably in aqueous solution, to produce coatings on metal surfaces, which coatings act as corrosion inhibitors and adhesion promotors. The amine moiety of these hydroxybenzylamines comprises secondary amine having alkyl substituents.
Deibig et al, in U.S. Pat. No. 4,447,477, teach a method for producing corrosion inhibiting coatings on metallic surfaces, whereby monomeric phenols and low molecular weight condensation products of formaldehyde and ammonia or amines are applied onto the pretreated surface and cured under heat.
None of the above references teach the diphenolamine oligomeric compound of the present invention, which compound is adapted to chelate metal ions to form a corrosion inhibiting coating on metal substrates.